Process for the preparation of a composition containing ethylene polymers, composition containing ethylene polymers and use thereof

ABSTRACT

Process for the preparation of a composition containing ethylene polymers comprising a polymer of melt index MI 2  of 5 to 1000 g/10 min and a polymer of melt index MI 5  of 0.01 to 2 g/10 min, the ratio of these indices being from 500 to 50,000 and the weight ratio of the two polymers being equal to (30 to 70):(70 to 30), according to which part of the ethylene, a catalyst derived from a transition metal having an intrinsic molecular weight distribution defined by an intrinsic M w /M n  ratio less than or equal to 10 and a deactivation constant less than or equal to 0.5 h −1 , and a cocatalyst are introduced into a first reactor, polymerization of the ethylene is carried out therein, a mixture comprising one of the polymers, the catalyst and the cocatalyst is drawn off from this reactor and the mixture and another part of the ethylene are introduced into a second reactor, which ethylene is polymerized to form the other polymer.

This is a division of application Ser. No. 08/571,684 filed Dec. 13,1995, Pat. No. 6,136,924 which in turn is a continuation application ofParent application Ser. No. 08/167,153 filed Dec. 16, 1993 ABN. Theentire disclosure of the prior application(s) is hereby incorporated byreference herein in its entirety.

The subject of the present invention is a process for the preparation ofa composition containing ethylene polymers, using a number of reactorsarranged in series. It relates, in particular, to a process for thepreparation of a composition containing ethylene polymers additionallycomprising an alpha-olefin.

A process for the preparation of a composition containing ethylenepolymers is described in Patent EP-22,376-B1 (Mitsui PetrochemicalIndustries), according to which at least two reactors are used inseries, a first part of the ethylene is polymerized in the presence of acatalyst in a first reactor of the series, a polymer and the catalystare drawn off from this reactor, these are made to move successivelyinto the other reactors into each of which another part of the ethyleneis delivered, which ethylene is polymerized, and a compositioncontaining ethylene polymers is recovered from the last reactor. Thisknown process uses, in each reactor, polymerization conditions differentfrom those used in the other reactors, so that, in each reactor, apolymer is produced which has a different viscosity—and consequently adifferent melt index—from those produced in the other reactors. Inparticular, the composition containing ethylene polymers obtained bythis known process comprises a first polymer having an intrinsicviscosity of 0.3 to 3 and a second polymer having an intrinsic viscosityof 1 to 12, the ratio between these viscosities being at least equal to1.5.

This known process does not make it possible to achieve a largedifference in the viscosities or the melt indices of the polymersproduced in the various reactors, so that it does not make it possibleto obtain polymers combining good use properties (characteristic ofpolymers with high melt indices) and good mechanical properties(characteristic of polymers with low melt indices).

Moreover, this known process is ill-suited to adjusting the molecularweight distribution of the final composition. Consequently, it does notallow access to a final composition suited to the implementation ofobjects by injection (composition having a molecular weight distributioncharacterized by an M_(w)/M_(n) ratio less than 10) nor to compositionswhich can be used for the manufacture of films by calendering(compositions in which the abovementioned M_(w)/M_(n) ratio is greaterthan 40).

This known process additionally has the disadvantage of causing, whenthe polymer of low viscosity or of high melt index is manufactured inone of the reactors in a hydrocarbon diluent in the presence ofhydrogen, rapid saturation of the diluent with hydrogen.

The present invention solves the disadvantages stated above by providinga novel process using a number of reactors which makes it possible toobtain a pronounced difference in the melt indices of the polymersobtained in the various reactors, which shows great flexibility inadjusting the molecular weight distribution of the final polymercomposition and which additionally makes it possible to produce apolymer of very high melt index in the presence of a hydrocarbon diluentand hydrogen, without risk of premature saturation of the diluent by thehydrogen.

Consequently, the invention relates to a process for the preparation ofa composition containing ethylene polymers comprising a polymer of highmelt index and a polymer of low melt index in at least two reactors,according to which part of the ethylene, a catalyst derived from atransition metal chosen from the elements of groups IIIB, IVB, VB andVIB of the periodic table and a cocatalyst are introduced into a firstreactor, polymerization of the ethylene is carried out therein, amixture comprising one of these polymers, the catalyst and thecocatalyst is drawn off from this reactor, the mixture and another partof the ethylene are introduced into a subsequent reactor, which ethyleneis polymerized to form the other polymer, the weight ratio of thepolymers being equal to (30 to 70):(70 to 30); according to theinvention, the catalyst has an intrinsic weight distribution defined byan intrinsic M_(w)/M_(n) ratio less than or equal to 10 and adeactivation constant less than or equal to 0.5 h⁻¹, and the polymer ofhigh melt index has a melt index MI₂, measured under a load of 2.16 kgat 190° C., of 5 to 1000 g/10 min and the polymer of low melt index hasa melt index MI₅, measured under a load of 5 kg at 190° C., of 0.01 to 2g/10 min, the ratio between these melt indices being from 500 to 50,000.

Intrinsic molecular weight distribution of a catalyst is understood todenote the molecular weight distribution of a polymer obtained in asingle polymerization stage and under constant polymerization conditionsin the presence of this catalyst. The intrinsic M_(w)/M_(n) ratio whichcharacterizes this intrinsic molecular weight distribution denotes theratio between the weight-average molecular mass (M_(w)) of the polymerthus obtained and the number-average molecular mass (M_(n)) of thispolymer, this ratio being measured by steric exclusion chromatographycarried out in 1,2,4-trichlorobenzene at 135° C. on a type 150 Cchromatograph from the company Waters.

Deactivation constant of a catalyst is understood to denote the angularcoefficient which characterizes the linear relationship between thelogarithm of the ratio of the polymerization rate and of the initialpolymerization rate, and the polymerization time, the polymerizationbeing carried out in the presence of this catalyst. The angularcoefficient is calculated using linear regression.

The melt index MI₂ (respectively MI₅) of a polymer denotes the flow rateof the molten polymer at 190° C., which flows through a die with adiameter of 2 mm and a length of 8 mm, under the effect of a pistonballasted with a mass of 2.16 kg (respectively 5 kg), this flow ratebeing expressed in g/10 min according to ASTM standard D 1238.

In the process according to the invention, ethylene is polymerized inthe presence of a catalyst. An essential characteristic of the processlies in the properties of the catalyst used. According to the invention,the catalyst has an intrinsic molecular weight distribution defined byan intrinsic M_(w)/M_(n) ratio at most equal to 10, preferably less than8, the values less than or equal to 7 being the most advantageous, forexample approximately 6.5 or 5. The intrinsic M_(w)/M_(n) ratio isusually greater than 3, the values greater than 4 being the most common.The catalyst used in the process according to the invention additionallyhas a deactivation constant less than or equal to 0.5 h⁻¹, preferably atmost equal to 0.3 h⁻¹, the values less than or equal to 0.2 h⁻¹, forexample of approximately 0.15 h⁻¹, being recommended. The deactivationconstant is generally greater than 0.05 h⁻¹, the values greater than orequal to 0.1 h⁻¹ being the most common.

The catalyst used in the process according to the invention can bechosen from the Ziegler-type catalysts, in particular those derived fromtitanium, and from metallocene-type catalysts, metallocene being acyclopentadienyl derivative of a transition metal, in particular ofzirconium.

There may be mentioned, as non-limiting examples of Ziegler-typecatalysts, the compounds comprising a transition metal chosen fromgroups IIIB, IVB, VB or VIB of the periodic table, magnesium and ahalogen obtained by mixing a magnesium compound with a compound of thetransition metal and a halogenated compound. The halogen can optionallyform an integral part of the magnesium compound or of the transitionmetal compound.

Mention may be made, as examples of metallocene-type catalysts, ofmetallocenes activated by an aluminoxane and ionic metallocenesactivated by an ionizing agent as described, for example, in PatentApplication EP-500,944-A1 (Mitsui Toatsu Chemicals).

Ziegler-type catalysts are preferred. Among these, those comprising atleast one transition metal chosen from groups IIIB, IVB, VB and VIB,magnesium and at least one halogen are very well suited. Good resultsare obtained with those comprising:

from 10 to 30% by weight of transition metal, preferably from 15 to 20%by weight, typically approximately 17% by weight,

from 20 to 60% by weight of halogen, the values from 30 to 50% by weight(for example, approximately 40% by weight) being preferred,

from 0.5 to 20% by weight of magnesium, usually from 1 to 10% by weight,for example approximately 5% by weight,

from 0.1 to 10% by weight of aluminium, generally from 0.5 to 5% byweight, the values from 1 to 3% by weight being the most common;

the balance generally consists of elements arising from the productsused for their manufacture, such as carbon, hydrogen and oxygen. Thetransition metal and the halogen are preferably titanium and chlorine.

In the process according to the invention, the polymerization is carriedout in the presence of a cocatalyst. It is possible to use anycocatalyst known in the art, especially compounds comprising at leastone aluminium-carbon chemical bond, such as optionally halogenatedorganoaluminium compounds, which can comprise oxygen or an element fromgroup I of the periodic table, and aluminoxanes. Mention may be made, asexamples of organoaluminium compounds, of trialkylaluminiums such astriethylaluminium, trialkenylaluminiums such as triisopropenylaluminium,aluminium mono- and dialkoxides such as diethylaluminium ethoxide, mono-and dihalogenated alkylaluminiums such as diethylaluminium chloride,alkylaluminium mono- and dihydrides such as dibutylaluminium hydride andorganoaluminium compounds comprising lithium such as LiAl (C₂H₅)₄.Organoaluminium compounds, especially those which are not halogenated,are well suited. Triethylaluminium and triisobutylaluminium areespecially advantageous.

In the process according to the invention, a plant is used comprising atleast two polymerization reactors arranged in series and connected toeach other. Each reactor is supplied with ethylene. The catalyst and thecocatalyst are introduced solely into the first reactor, in whichethylene is polymerized until a polymer is obtained which has thecharacteristics specific to the polymerization conditions of thisreactor. A mixture arising from the first reactor and comprising thepolymer obtained in the latter, the catalyst and the cocatalyst isintroduced, preferably continuously, into the second reactor. Ethylene,which is introduced into this second reactor, is polymerized thereinusing the catalyst and cocatalyst arising from the first reactor andpolymerization conditions (temperature, concentration of transfer agent,concentration of optional comonomer) are used in this second reactorwhich are different from those used in the first reactor. Thus, thepolymer produced in the second reactor has a melt index different fromthat produced in the first, and the overall polymer compositioncollected from the second reactor combines characteristics specific tothe operating conditions of the first reactor and characteristicsspecific to the operating conditions of the second reactor.

In the process according to the invention, the polymer of high meltindex and the polymer of low melt index can be prepared in any order.

The plant can obviously comprise more than two reactors connected inseries. In this case, the first reactor of the series is supplied withthe catalyst and the cocatalyst, and each reactor is supplied withethylene and with the mixture arising from the preceding reactor of theseries, this mixture comprising the catalyst, the cocatalyst and amixture of the polymers produced in the preceding reactors of theseries.

In the case where the plant comprises more than two reactors in series,the polymer of high melt index and the polymer of low melt index asdefined above can be produced in two adjacent or non-adjacent reactorsin the series. In this specific case of the use of the process accordingto the invention, it is possible to produce, in the other reactors ofthe series, operating conditions under which there is produced either apolymer of melt index MI₂ less than 5, from 5 to 1000 or greater than1000 or a melt index MI₅ less than 0.01, from 0.01 to 2 or greater than2, the melt indices MI₂ and MI₅ having been defined above.

The reaction is preferably limited to two reactors.

In a first embodiment of the process according to the invention, theformation of the polymer of high melt index precedes that of the polymerof low melt index. This embodiment proves to be particularlyadvantageous when it is desired to obtain a composition containingethylene polymers which can be used for the manufacture of shapedobjects whose surface is free of imperfections such as hard points.

In a second embodiment of the process according to the invention, atleast one of the reactors is supplied with hydrogen which acts astransfer agent modulating the melt index of the polymer produced in thisreactor. The hydrogen partial pressure in the reactor(s) isadvantageously from 0.001 to 2 MPa, more particularly from 0.002 to 1.5MPa, preferably from 0.005 to 1.3 MPa, the ratio between the hydrogenand ethylene partial pressures generally not exceeding 5, preferably notexceeding 3 and being, for example, between 0.01 and 2.5.

In a variant of this second embodiment, hydrogen is introducedcontinuously into all the reactors, the ratio between the ethylene andhydrogen partial pressures in the first reactor being different fromthat used in the other reactors. In this variant, it is important tokeep these ratios constant in each reactor throughout the duration ofthe polymerization. The quotient of these two ratios is advantageouslygreater than 20, preferably than 40; it is desirable that it does notexceed 300, for example 200. A quotient selected from 45 to 175 isparticularly well suited.

This embodiment of the process according to the invention has theadvantageous distinctive characteristic of making it possible to obtaina polymer with a very high melt index in the presence of a hydrocarbondiluent while avoiding rapid saturation of the hydrocarbon diluent bythe hydrogen.

In the process according to the invention, the polymerization procedurein the reactors can be selected from the solution, suspension orgas-phase processes, irrespective of the properties of the polymer whichit is desired to prepare therein and of the choice of the process usedin the other reactor. For example, it is possible to carry out thepolymerization in two gas-phase reactors, or in a first reactor insuspension and in a second reactor in the gas phase or in reverse order.The polymerization is preferably carried out in suspension in tworeactors.

In the case of a suspension polymerization, the latter is generallycarried out in a hydrocarbon diluent which is inert with respect to thecatalyst, cocatalyst and polymer produced (such as liquid aliphatic,cycloaliphatic and aromatic hydrocarbons), at a temperature such that atleast 50% (preferably at least 70%) of the polymer formed is insolubletherein. Preferred diluents are linear alkanes such as n-butane,n-hexane and n-heptane, or branched alkanes such as isobutane,isopentane, isooctane and 2,2,-dimethylpropane, or cycloalkanes such ascyclopentane and cyclohexane or their mixtures. The polymerizationtemperature is generally chosen from 20 to 200° C., preferably from 50to 100° C. The ethylene partial pressure is most often chosen from 0.1to 5 MPa, preferably from 0.2 to 2 MPa, more particularly from 0.4 to1.5 MPa.

In the process according to the invention, it is optionally possible tosupply the second reactor and/or, if appropriate, at least one of thefollowing reactors with fresh catalyst and/or cocatalyst. However, it ispreferable to introduce the catalyst and the cocatalyst exclusively intothe first reactor.

In a specific embodiment of the process according to the invention, analpha-olefin is additionally introduced into at least one of thereactors 50 as to manufacture, in this reactor, a copolymer of ethyleneand of this alpha-olefin. The alpha-olefin can be selected fromolefinically unsaturated monomers comprising from 3 to 8 carbon atoms,for example propylene, 1-butene, 1-pentene, 3-methyl-1-butene, 1-hexene,3- and 4-methyl-1-pentenes and 1-octene. Other examples of alpha-olefinsare diolefins comprising from 4 to 18 carbon atoms, preferablynon-conjugated aliphatic diolefins such as 4-vinylcyclohexene and1,5-hexadiene, alicyclic diolefins having an endocyclic bridge such asdicyclopentadiene or methylene- and ethylidenenorbornene, and conjugatedaliphatic diolefins such as 1,3-butadiene, isoprene and 1,3-pentadiene.Preferred alpha-olefins are propylene, 1-butene, 1-hexene, 1-octene and1,5-hexadiene. Good results are obtained with 1-butene and 1-hexene.

In this specific embodiment of the process according to the invention,the alpha-olefin is generally introduced into that reactor in which thepolymer of low melt index is produced, in an amount adjusted so thatthis polymer comprises from 0.5 to 20% by weight of alpha-olefin,preferably from 1 to 10% by weight, for example 2% by weight. As analternative, part of the alpha-olefin can also be introduced into theother reactor, in a restricted amount so that the alpha-olefin contentof the polymer of high melt index does not exceed 5% by weight,preferably 3% by weight, for example 1% by weight; the alpha-olefincontent of the polymer of high melt index is usually at least equal to0.1%.

The process according to the invention applies to the preparation ofcompositions containing ethylene polymers which can comprise one or anumber of ethylene homopolymers and/or one or a number of ethylenecopolymers.

The process according to the invention makes it possible to obtaincompositions containing polymers of ethylene, and optionally ofalpha-olefins, in which each individual polymer has a melt indexsufficiently different from that of the other or of each of the otherpolymers, in order to benefit simultaneously from properties favourableto use characteristic of a polymer of high melt index and goodmechanical properties characteristic of a polymer of low melt index.

The process according to the invention, moreover, has great flexibilityin adjusting the molecular weight distribution in the final composition.Thus, the process according to the invention makes it possible tomanufacture a wide range of compositions containing ethylene polymersranging from those suited to the use of injection-moulded objects tothose which can be used for the manufacture of films by extrusion orcalendering.

Additionally, the process according to the invention makes it possibleto obtain compositions containing polymers of ethylene and optionally ofalpha-olefins comprising an alpha-olefin in a variable amount which canreach 10% by weight, preferably from 0.5 to 6% by weight, for exampleapproximately 1% by weight.

The process according to the invention proves to be particularlyoutstanding for the manufacture of compositions containing polymers ofethylene and optionally of alpha-olefins which can be used for themanufacture of objects having a high resistance to cracking under stressand whose surface is free of imperfections such as hard points.

The invention consequently also relates to compositions containingethylene polymers having the properties stated above, these compositionscomprising, on the one hand, a first polymer having a melt index MI₂ of5 to 1000 g/10 min, preferably from 10 to 500 g/10 min, and, on theother hand, a second polymer having a melt index MI₅ of 0.01 to 2 g/10min, preferably from 0.03 to 1 g/10 min, in particular from 0.05 to 0.7g/10 min, the ratio of these melt indices being from 500 to 50,000,preferably from 1000 to 10,000. The weight ratio of these two polymersis generally equal to (30 to 70):(70 to 30), preferably to (40 to60):(60 to 40), for example to (42 to 58):(58 to 42). The firstabovementioned polymer can optionally comprise an alpha-olefin in anamount at most equal to 5% by weight and the second polymer comprisesfrom 0.5 to 20% by weight of an alpha-olefin. Advantageous compositionscomprise an ethylene homopolymer of high melt index and an ethylenecopolymer of low melt index containing, for example, from 0.5 to 6% byweight of an alpha-olefin. The compositions according to the inventionhave a molecular weight distribution defined by an M_(w)/M_(n) ratiowhich can vary from 5 to 70, in particular from 7 to 50, for examplefrom 10 to 40. Additionally, the compositions according to the inventionhave a melt index MI₅ of 0.1 to 10 g/10 min, in particular from 0.5 to 5g/10 min, and a dynamic viscosity ρ, expressed in dpa.s and measured ata rate gradient of 100 s⁻¹ at 190° C., corresponding to the relationship$0.652 \geq {\frac{\left\lbrack {{\log \left( {177470/{MI}_{5}} \right)} - {\log \quad \eta}} \right\rbrack}{2 - {\log \left( {2.53 \times {MI}_{5}} \right)}}.}$

The compositions according to the invention commonly contain from 0.5 to10% by weight of an alpha-olefin, preferably from 1 to 6% by weight.

The compositions according to the invention find a particularlyadvantageous use in a wide range of industrial applications, from thefact that they combine good use properties and good mechanicalproperties such as impact resistance and resistance to cracking understress. The compositions according to the invention are suitable forbeing used by any conventional process for converting plastics and moreparticularly by extrusion, blown extrusion, thermoforming-extrusion,calendering and injection processes. These compositions are suitable forthe manufacture of shaped objects such as films, sheets, panels,containers, bags and sachets; they are particularly well suited to themanufacture of pipes.

The present invention consequently also relates to the use of thecompositions described above for the manufacture of pipes.

The examples whose description follows are used to illustrate theinvention.

The meaning of the symbols used in these examples, the units expressingthe quantities mentioned and the methods for measuring these quantitiesare explained below.

MI₂ =melt index of a polymer denoting the flow rate of the moltenpolymer at 190° C., which flows through a die with a diameter of 2 mmand a length of 8 mm, under the effect of a piston ballasted with a massof 2.16 kg, this flow rate being expressed in g/10 min, according toASTM standard D 1238.

MI₅=melt index of a polymer (or of a polymer composition) denoting theflow rate of the molten polymer (or of the molten composition) at 190°C., which flows through a die with a diameter of 2 mm and a length of 8mm, under the effect of a piston ballasted with a mass of 5 kg, thisflow rate being expressed in g/10 min, according to ASTM standard D1238.

M_(w)/M_(n)=ratio between the weight-average molecular mass (M_(w)) of apolymer (or of a polymer composition) and the number-average molecularmass (M_(n)) of this polymer (or of this composition), measured bysteric exclusion chromatography carried out in 1,2,4-trichlorobenzene at135° C. on a type 150 C chromatograph from the company Waters.

ρ=dynamic viscosity of a polymer (or of a polymer composition) expressedin dpa.s and measured at a rate gradient of 100 s⁻¹ at 190° C.

K_(d)=deactivation constant of a catalyst expressed in h⁻¹, which is theangular coefficient which characterizes the linear relationship betweenthe logarithm of the ratio of the polymerization rate and of the initialpolymerization rate, and the polymerization time, the polymerizationbeing carried out in the presence of this catalyst. The angularcoefficient is calculated using linear regression.

α=catalytic activity in grams of insoluble polymer obtained per hour andper gram of catalyst and divided by the molar fraction of ethylene inthe diluent.

EXAMPLE 1 (In Accordance With the Invention)

A. Preparation of the catalyst

Magnesium diethoxide is reacted with titanium tetrabutoxide for 4 hoursat 150° C. in amounts such that the molar ratio of titanium to magnesiumis equal to 2. The reaction product thus obtained was then chlorinatedand precipitated by bringing the latter into contact with anethylaluminium dichloride solution for 90 minutes at 45° C. The solidthus obtained, collected from the suspension, comprised (% by weight):

Ti : 17

Cl : 41

Al : 2

Mg : 5.

B. Polymerization of Ethylene in a Single Reactor

1 litre of hexane and 1 mmol of triethylaluminium were introduced into a1.5 litre autoclave, equipped with a stirrer. The temperature was thenraised to 85° C., which was maintained constant throughout thepolymerization time. A single charge of hydrogen at a pressure of 0.4MPa and ethylene were then introduced therein. 7 mg of the solidcatalyst obtained in A were then injected therein. The ethylene partialpressure was kept constant at a value of 1 MPa for 1 hour. The autoclavewas then degassed and cooled. The polyethylene collected from theautoclave had an M_(w)/M_(n) ratio of 6.7 and the catalyst had a K_(d)of 0.15.

C. Polymerization of Ethylene in Two Reactors

The polymerization process in two successive reactors was simulated in asingle reactor in two stages separated by intermediate pressure releaseand reinitialization of the operating parameters.

Polymerization of a First Polymer (i):

2 litres of hexane and 2 mmol of triethylaluminium were introduced intoa 5 litre autoclave, equipped with a stirrer. The temperature was thenraised to 85° C., which was kept constant throughout the polymerizationtime. A single charge of hydrogen at a pressure of 1.3 MPa and ethylenewere then introduced therein. The ethylene partial pressure was keptconstant at a value of 0.6 MPa. 22 mg of the solid catalyst obtained inA were then injected therein. After 73 minutes, the autoclave wasdegassed. 200 g of polymer (i) were obtained. The catalyst had anactivity a of 10.3.

Polymerization of Second Polymer (ii):

200 ml of hexane were readded to the autoclave. The temperature wasbrought to 75° C. and kept constant throughout the polymerization time.A hydrogen charge at a pressure of 0.08 MPa, ethylene and a butenecharge were then introduced so as to obtain a butene/ethylene molarratio in the liquid phase of 0.38. The ethylene partial pressure waskept constant at a value of 0.4 MPa until an additional amount of 169 gof polymer (ii) was obtained. After degassing, 369 g of a compositioncontaining polymers (i) and (ii) were collected from the autoclave.

The Catalyst had an Activity α of 7.3.

The following results were obtained:

Polymer (i):

MI₂=168

Polymer (ii):

MI₅=0.21

Composition Comprising the Polymers (i) and (ii):

MI₅=15.9

ρ=6700

M_(w)/M_(n)=21.

EXAMPLE 2 For Reference

In this example, a catalyst was manufactured having an intrinsicdistribution defined by an M_(w)/M_(n) ratio greater than 10, which wasthen used in a process for the polymerization of ethylene in tworeactors.

A. Preparation of the Catalyst

Magnesium diethoxide, titanium tetrabutoxide and zirconium tetrabutoxidewere reacted for 4 hours at 150° C. in amounts such that the Ti/Mg molarratio is equal to 0.6 and such that the Zr/Ti molar ratio is equal to1.2. The reaction product thus obtained was then chlorinated andprecipitated by bringing the latter into contact with a solution ofisobutylaluminium dichloride, first at 45° C. and then at 60° C. Thesolid thus obtained, collected from the suspension, comprised (% byweight):

Ti : 6

Zr : 12

Cl : 50

Al : 2

Mg : 5.

B. Polymerization of Ethylene in a Single Reactor

The operations of Example 1 (B) were repeated under the followingoperating conditions:

initial hydrogen partial pressure: 1.2 MPa

ethylene partial pressure: 0.6 MPa

amount of catalyst used: 12 mg

duration of polymerization: 42 min

amount of polyethylene produced: 60 g.

The polymer thus obtained had an M_(w)/M_(n) ratio of 19 and thecatalyst had a K_(d) of 1.

C. Polymerization of Ethylene in Two Reactors

The operations of Example 1 (C) were repeated under the followingoperating conditions:

Polymerization of a First Polymer (i):

polymerization temperature: 85° C.

initial hydrogen partial pressure: 1.2 MPa

ethylene partial pressure: 0.6 MPa

amount of catalyst used: 12 mg

amount of polyethylene produced: 54 g.

The Catalyst had an Activity α of 19.3.

Polymerization of Second Polymer (ii):

polymerization temperature: 70° C.

initial hydrogen partial pressure: 0.2 MPa

ethylene partial pressure: 0.6 MPa

butene/ethylene molar ratio: 0.28

amount of polyethylene produced in the second stage: 80 g

total amount of polyethylene produced: 134 g.

The following results were obtained:

Polymer (i):

MI₂=1.4

Polymer (ii):

MI₅=0.03

Composition Comprising the Polymers (i) and (ii):

MI₅=0.09

ρ=23500

M_(w)/M_(n)=18.

A comparison of the results of Example 2 with those of Example 1 makesapparent the progress brought about by the invention as regards thedifference between the melt indices of the polymers (i) and (ii)obtained in the two reactors.

What is claimed is:
 1. A pipe comprising a composition comprisingethylene polymers comprising; a first ethylene polymer having a meltindex MI₂, measured at 190° C. under a load of 2.16 kg, of 5 to 1000g/10 min and optionally comprising an alpha-olefin in an amount at mostequal to 5% by weight; and a second ethylene polymer having a melt indexMI₅, measured at 190° C. under a load of 5 kg, of 0.01 to 2 g/10 min,and comprising from 0.5 to 20% by weight of an alpha-olefin, wherein theratio of the melt indices is from 500 to 50,000, the weight ratio of thepolymers is equal to (30 to 70):(70 to 30), and wherein the compositionhas the following characteristics: a molecular weight distributiondefined by an M_(w)/M_(n) ratio of 5 to 70, a melt index MI₅ of 0.1 to10 g/10 min and a dynamic viscosity η, expressed in dPa.s and measuredat a rate gradient of 100 s⁻¹ at 190 ° C., corresponding to therelationship$0.652 \geq {\frac{\left\lbrack {{\log \left( {177470/{MI}_{5}} \right)} - {\log \quad \eta}} \right\rbrack}{2 - {\log \left( {2.53\quad {MI}_{5}} \right)}}.}$


2. The pipe of claim 1, wherein the composition comprises from 0.5 to10% by weight of an alpha-olefin.
 3. The pipe of claim 1, wherein thepolymer of high melt index is an ethylene homopolymer and the polymer oflow melt index is an ethylene copolymer having an alpha-olefin contentof 0.5 to 6% by weight.